Process of forming a grid electrode on the front-side of a silicon wafer

ABSTRACT

A process of forming a front-grid electrode on a silicon wafer having an ARC layer, comprising the steps:
     (1) printing and drying a metal paste A comprising an inorganic content comprising 0.5 to 8 wt.-% of glass frit and having fire-through capability, wherein the metal paste A is printed on the ARC layer to form a bottom set of thin parallel finger lines,   (2) printing and drying a metal paste B comprising an inorganic content comprising 0.2 to 3 wt.-% of glass frit over the bottom set of finger lines, wherein the metal paste B is printed in a grid pattern which comprises (i) thin parallel finger lines forming a top set of finger lines superimposing the bottom set of finger lines and (ii) busbars intersecting the finger lines at right angle, and   (3) firing the double-printed silicon wafer,
 
wherein the inorganic content of metal paste B contains less glass frit plus optionally present other inorganic additives than the inorganic content of metal paste A.

FIELD OF THE INVENTION

The present invention is directed to a process of forming a gridelectrode on the front-side of a silicon wafer.

TECHNICAL BACKGROUND OF THE INVENTION

A conventional solar cell structure with a p-type base has a negativeelectrode that is typically on the front-side or illuminated side of thecell and a positive electrode on the back-side. It is well known thatradiation of an appropriate wavelength falling on a p-n junction of asemiconductor body serves as a source of external energy to generateelectron-hole pairs in that body. The potential difference that existsat a p-n junction, causes holes and electrons to move across thejunction in opposite directions, thereby giving rise to flow of anelectric current that is capable of delivering power to an externalcircuit. Most solar cells are in the form of a silicon wafer that hasbeen metallized, i.e., provided with metal contacts which areelectrically conductive.

Most electric power-generating solar cells currently used are siliconsolar cells. Electrodes in particular are made by using a method such asscreen printing from metal pastes.

The production of a silicon solar cell typically starts with a p-typesilicon substrate in the form of a silicon wafer on which an n-typediffusion layer of the reverse conductivity type is formed by thethermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride(POCl₃) is commonly used as the gaseous phosphorus diffusion source,other liquid sources are phosphoric acid and the like. In the absence ofany particular modification, the diffusion layer is formed over theentire surface of the silicon substrate. The p-n junction is formedwhere the concentration of the p-type dopant equals the concentration ofthe n-type dopant; conventional cells that have the p-n junction closeto the illuminated side, have a junction depth between 0.05 and 0.5 μm.

After formation of this diffusion layer excess surface glass is removedfrom the rest of the surfaces by etching by an acid such as hydrofluoricacid.

Next, an ARC layer (antireflective coating layer) of TiO_(x), SiO_(x),TiO_(x)/SiO_(x), or, in particular, SiN_(x) or Si₃N₄ is formed on then-type diffusion layer to a thickness of between 0.05 and 0.1 μm by aprocess, such as, for example, plasma CVD (chemical vapor deposition).

A conventional solar cell structure with a p-type base typically has anegative grid electrode on the front-side of the cell and a positiveelectrode on the back-side. The grid electrode is typically applied byscreen printing and drying a front-side silver paste (front electrodeforming silver paste) on the ARC layer on the front-side of the cell.The front-side grid electrode is typically screen printed in a so-calledH pattern which comprises (i) thin parallel finger lines (collectorlines) and (ii) two busbars intersecting the finger lines at rightangle. In addition, a back-side silver or silver/aluminum paste and analuminum paste are screen printed (or some other application method) andsuccessively dried on the back-side of the substrate. Normally, theback-side silver or silver/aluminum paste is screen printed onto thesilicon wafer's back-side first as two parallel busbars or as rectangles(tabs) ready for soldering interconnection strings (presoldered copperribbons). The aluminum paste is then printed in the bare areas with aslight overlap over the back-side silver or silver/aluminum. In somecases, the silver or silver/aluminum paste is printed after the aluminumpaste has been printed. Firing is then typically carried out in a beltfurnace for a period of 1 to 5 minutes with the wafer reaching a peaktemperature in the range of 700 to 900° C. The front grid electrode andthe back electrodes can be fired sequentially or cofired.

The aluminum paste is generally screen printed and dried on theback-side of the silicon wafer. The wafer is fired at a temperatureabove the melting point of aluminum to form an aluminum-silicon melt,subsequently, during the cooling phase, an epitaxially grown layer ofsilicon is formed that is doped with aluminum. This layer is generallycalled the back surface field (BSF) layer. The aluminum paste istransformed by firing from a dried state to an aluminum back electrode.The back-side silver or silver/aluminum paste is fired at the same time,becoming a silver or silver/aluminum back electrode. During firing, theboundary between the back-side aluminum and the back-side silver orsilver/aluminum assumes an alloy state, and is connected electrically aswell. The aluminum electrode accounts for most areas of the backelectrode, owing in part to the need to form a p+ layer. A silver orsilver/aluminum back electrode is formed over portions of the back-side(often as 2 to 6 mm wide busbars) as an electrode for interconnectingsolar cells by means of pre-soldered copper ribbon or the like. Inaddition, the front-side silver paste printed as front-side gridelectrode sinters and penetrates through the ARC layer during firing,and is thereby able to electrically contact the n-type layer. This typeof process is generally called “firing through”.

It has been found that the electrical efficiency of a silicon solar cellcan be improved, where the finger lines of the front-side grid electrodeare double-printed and where the metal pastes used for the first and thesecond printing differ in their content of glass frit plus optionallypresent other inorganic additives. Adhesion between the printed layersis also improved compared to simply printing the same metal paste twice.

In the present description and the claims the term “content of glassfrit plus optionally present other inorganic additives” is used. Itmeans the inorganic components of a metal paste without the metal.

SUMMARY OF THE INVENTION

The present invention relates to a process of forming a grid electrodeon the front-side of a silicon wafer having a p-type region, an n-typeregion, a p-n junction and an ARC layer on said front-side, comprisingthe steps:

(1) printing and drying a metal paste A having fire-through capabilityon the ARC layer, wherein the metal paste A is printed as thin parallelfinger lines forming a bottom set of finger lines,

(2) printing and drying a metal paste B over the bottom set of fingerlines, wherein the metal paste B is printed in a grid pattern whichcomprises (i) thin parallel finger lines forming a top set of fingerlines superimposing the bottom set of finger lines and (ii) two or moreparallel busbars intersecting the finger lines at right angle, and

(3) firing the double-printed silicon wafer,

wherein the metal paste A comprises an organic vehicle and an inorganiccontent comprising (a1) at least one electrically conductive metalpowder selected from the group consisting of silver, copper and nickel,and (a2) 0.5 to 8 wt.-% (weight-%), preferably 1 to 3 wt.-% of glassfrit,wherein the metal paste B comprises an organic vehicle and an inorganiccontent comprising (b1) at least one electrically conductive metalpowder selected from the group consisting of silver, copper and nickel,and (b2) 0.2 to 3 wt.-%, preferably 0.2 to 2 wt.-% of glass frit, andwherein the inorganic content of metal paste B contains less glass fritplus optionally present other inorganic additives than the inorganiccontent of metal paste A.

DETAILED DESCRIPTION OF THE INVENTION

In the description and the claims the term “fire-through capability” isused. A metal paste with fire-through capability is one that firesthrough an ARC layer making electrical contact with the surface of thesilicon substrate. Correspondingly, a metal paste with poor or even nofire through capability makes only poor or even no electrical contactwith the silicon substrate upon firing.

In step (1) of the process of the present invention a metal paste A withfire-through capability is printed on the ARC layer on the front-side ofa silicon wafer. The silicon wafer is a conventional mono- orpolycrystalline silicon wafer as is conventionally used for theproduction of silicon solar cells; it has a p-type region, an n-typeregion and a p-n junction. The silicon wafer has an ARC layer, forexample, of TiO_(x), SiO_(x), TiO_(x)/SiO_(x), or, in particular,SiN_(x) or Si₃N₄ on its front-side. Such silicon wafers are well knownto the skilled person; for brevity reasons reference is made to thesection “TECHNICAL BACKGROUND OF THE INVENTION”. The silicon wafer mayalready be provided with the conventional back-side metallizations, i.e.with a back-side aluminum paste and a back-side silver or back-sidesilver/aluminum paste as described above in the section “TECHNICALBACKGROUND OF THE INVENTION”. Application of the back-side metal pastesmay be carried out before or after the front-side grid electrode isfinished. The back-side pastes may be individually fired or cofired oreven be cofired with the front-side metal pastes printed on the ARClayer in steps (1) and (2).

Metal Paste A

Metal paste A is a thick film conductive composition with fire-throughcapability. It comprises an organic vehicle and an inorganic contentcomprising (a1) at least one electrically conductive metal powderselected from the group consisting of silver, copper and nickel, and(a2) 0.5 to 8 wt.-%, preferably 1 to 3 wt.-% of glass frit.

Metal paste A comprises an organic vehicle. A wide variety of inertviscous materials can be used as organic vehicle. The organic vehiclemay be one in which the particulate constituents (electricallyconductive metal powder, glass frit) are dispersible with an adequatedegree of stability. The properties, in particular, the rheologicalproperties, of the organic vehicle may be such that they lend goodapplication properties to the metal paste, including: stable dispersionof insoluble solids, appropriate viscosity and thixotropy for printing,in particular, for screen printing, appropriate wettability of the ARClayer on the front-side of the silicon wafer and of the paste solids, agood drying rate, and good firing properties. The organic vehicle usedin metal paste A may be a nonaqueous inert liquid. The organic vehiclemay be an organic solvent or an organic solvent mixture; in anembodiment, the organic vehicle may be a solution of organic polymer(s)in organic solvent(s). Use can be made of any of various organicvehicles, which may or may not contain thickeners, stabilizers and/orother common additives. In an embodiment, the polymer used asconstituent of the organic vehicle may be ethyl cellulose. Otherexamples of polymers which may be used alone or in combination includeethylhydroxyethyl cellulose, wood rosin, phenolic resins andpoly(meth)acrylates of lower alcohols. Examples of suitable organicsolvents comprise ester alcohols and terpenes such as alpha- orbeta-terpineol or mixtures thereof with other solvents such as kerosene,dibutylphthalate, diethylene glycol butyl ether, diethylene glycol butylether acetate, hexylene glycol and high boiling alcohols. In addition,volatile organic solvents for promoting rapid hardening after printapplication of metal paste A can be included in the organic vehicle.Various combinations of these and other solvents may be formulated toobtain the viscosity and volatility requirements desired.

The ratio of organic vehicle in metal paste A to the inorganic content(inorganic components; electrically conductive metal powder plus glassfrit plus optionally present other inorganic additives) is dependent onthe method of printing metal paste A and the kind of organic vehicleused, and it can vary. Usually, metal paste A will contain 58 to 95wt.-% of inorganic components and 5 to 42 wt.-% of organic vehicle.

The inorganic content of metal paste A comprises (a1) at least oneelectrically conductive metal powder selected from the group consistingof silver, copper and nickel, and (a2) 0.5 to 8 wt.-%, preferably 1 to 3wt.-% of glass frit. The inorganic content may further comprise otherinorganic additives, for example, solid oxides or compounds capable offorming solid oxides during firing of metal paste A. In an embodiment,the inorganic content of metal paste A consists of (a1) at least oneelectrically conductive metal powder selected from the group consistingof silver, copper and nickel, and (a2) 0.5 to 8 wt.-%, preferably 1 to 3wt.-% of glass frit.

Metal paste A comprises at least one electrically conductive metalpowder selected from the group consisting of silver, copper and nickel.Silver powder is preferred. The metal or silver powder may be uncoatedor at least partially coated with a surfactant. The surfactant may beselected from, but is not limited to, stearic acid, palmitic acid,lauric acid, oleic acid, capric acid, myristic acid and linolic acid andsalts thereof, for example, ammonium, sodium or potassium salts.

The average particle size of the electrically conductive metal powderor, in particular, silver powder is in the range of, for example, 0.5 to5 μm. The total content of the electrically conductive metal powder or,in particular, silver powder in metal paste A is, for example, 50 to 92wt.-%, or, in an embodiment, 65 to 84 wt.-%.

In the description and the claims the term “average particle size” isused. It means the mean particle diameter (d50) determined by means oflaser scattering. All statements made in the present description and theclaims in relation to average particle sizes relate to average particlesizes of the relevant materials as are present in the metal pastes A andB.

In general metal paste A comprises only the at least one electricallyconductive metal powder selected from the group consisting of silver,copper, and nickel. However, it is possible to replace a smallproportion of the electrically conductive metal selected from the groupconsisting of silver, copper and nickel by one or more other particulatemetals. The proportion of such other particulate metal(s) is, forexample, 0 to 10 wt. %, based on the total of particulate metalscontained in metal paste A.

As already mentioned, metal paste A comprises glass frit as inorganicbinder. The average particle size of the glass frit is in the range of,for example, 0.5 to 4 μm.

The preparation of the glass frit is well known and consists, forexample, in melting together the constituents of the glass in the formof the oxides of the constituents and pouring such molten compositioninto water to form the frit. As is well known in the art, heating may beconducted to a peak temperature and for a time such that the meltbecomes entirely liquid and homogeneous.

The glass may be milled in a ball mill with water or inert lowviscosity, low boiling point organic liquid to reduce the particle sizeof the frit and to obtain a frit of substantially uniform size. It maythen be settled in water or said organic liquid to separate fines andthe supernatant fluid containing the fines may be removed. Other methodsof classification may be used as well.

Metal paste A is a viscous composition, which may be prepared bymechanically mixing the electrically conductive metal powder(s) and theglass frit with the organic vehicle. In an embodiment, the manufacturingmethod power mixing, a dispersion technique that is equivalent to thetraditional roll milling, may be used; roll milling or other mixingtechnique can also be used.

Metal paste A can be used as such or may be diluted, for example, by theaddition of additional organic solvent(s); accordingly, the weightpercentage of all the other constituents of metal paste A may bedecreased.

In step (1) of the process of the present invention metal paste A isprinted, in particular, screen printed as thin parallel finger linesforming a bottom set of finger lines. The parallel finger lines have adistance between each other of, for example, 2 to 5 mm, a dry layerthickness of, for example, 3 to 30 μm and a width of, for example, 25 to150 μm.

The printed metal paste A is dried, for example, for a period of 1 to100 minutes with the silicon wafer reaching a peak temperature in therange of 100 to 300° C. Drying can be carried out making use of, forexample, belt, rotary or stationary driers, in particular, IR (infrared)belt driers.

In step (2) of the process of the present invention a metal paste B isprinted, in particular, screen printed in a grid pattern which comprises(i) thin parallel finger lines forming a top set of finger linessuperimposing the bottom set of finger lines and (ii) two or moreparallel busbars intersecting the finger lines at right angle.

Metal Paste B

Metal paste B is a thick film conductive composition that may or may nothave or may have only poor fire-through capability. In a preferredembodiment it does not have fire-through capability. In anotherpreferred embodiment it has only poor fire-through capability. Itcomprises an organic vehicle and an inorganic content comprising (b1) atleast one electrically conductive metal powder selected from the groupconsisting of silver, copper and nickel, and (b2) 0.2 to 3 wt.-%,preferably 0.2 to 2 wt.-% of glass frit.

It is essential that the inorganic content of metal paste B containsless glass frit plus optionally present other inorganic additives thanthe inorganic content of metal paste A. In an embodiment, the inorganiccontent of metal paste B contains less glass frit than the inorganiccontent of metal paste A.

Metal paste B comprises an organic vehicle. With regard to the organicvehicle the same applies as already mentioned above in connection withthe organic vehicle in metal paste A.

Metal paste B comprises at least one electrically conductive metalpowder selected from the group consisting of silver, copper and nickel.Silver powder is preferred. The metal or silver powder may be uncoatedor at least partially coated with a surfactant. The surfactant may beselected from, but is not limited to, stearic acid, palmitic acid,lauric acid, oleic acid, capric acid, myristic acid and linolic acid andsalts thereof, for example, ammonium, sodium or potassium salts.

The average particle size of the electrically conductive metal powderor, in particular, silver powder is in the range of, for example, 0.5 to5 μm. The total content of the electrically conductive metal powder or,in particular, silver powder in metal paste B is, for example, 50 to 92wt.-%, or, in an embodiment, 65 to 84 wt.-%.

In general metal paste B comprises only the at least one electricallyconductive metal powder selected from the group consisting of silver,copper, and nickel. However, it is possible to replace a smallproportion of the electrically conductive metal selected from the groupconsisting of silver, copper and nickel by one or more other particulatemetals. The proportion of such other particulate metal(s) is, forexample, 0 to 10 wt. %, based on the total of particulate metalscontained in metal paste B.

As already mentioned, metal paste B comprises glass frit as inorganicbinder. The average particle size of the glass frit is in the range of,for example, 0.5 to 4 μm.

In the preferred cases of a metal paste B without or with only poorfire-through capability, the glass frit contained in metal paste B mayconsist of at least one glass frit selected from the group consisting of(i) lead-containing glass frits with a softening point temperature(glass transition temperature, determined by differential thermalanalysis DTA at a heating rate of 10 K/min) in the range of 571 to 636°C. and containing 53 to 57 wt.-% of PbO, 25 to 29 wt.-% of SiO₂, 2 to 6wt.-% of Al₂O₃ and 6 to 9 wt.-% of B₂O₃, (ii) lead-free glass frits witha softening point temperature in the range of 550 to 611° C. andcontaining 11 to 33 wt.-% of SiO₂, >0 to 7 wt.-%, in particular 5 to 6wt.-% of Al₂O₃ and 2 to 10 wt.-% of B₂O₃, and (iii) lead-free glassfrits with a softening point temperature in the range of 550 to 611° C.and containing 40 to 73 wt.-% of Bi₂O₃, 11 to 33 wt.-% of SiO₂, >0 to 7wt.-%, in particular 5 to 6 wt.-% of Al₂O₃ and 2 to 10 wt.-% of B₂O₃. Inother words, here, the at least one glass frit may comprise glass fritof type (i) and/or of type (ii) and/or of type (iii); the ratio betweenthe different glass frit types may be any. In case of the lead-freeglass frit of type (ii), the weight percentages of SiO₂, Al₂O₃ and B₂O₃do not total 100 wt.-% and the missing wt.-% are in particularcontributed by one or more other oxides, for example, alkali metaloxides like Na₂O, alkaline earth metal oxides like MgO and metal oxideslike Bi₂O₃, TiO₂ and ZnO. In case of the lead-free glass frit of type(iii) which represents a particular embodiment of the lead-free glassfrit of type (ii), the weight percentages of Bi₂O₃, SiO₂, Al₂O₃ and B₂O₃may or may not total 100 wt.-%; in case they do not total 100 wt.-% themissing wt.-% may in particular be contributed by one or more otheroxides, for example, alkali metal oxides like Na₂O, alkaline earth metaloxides like MgO and metal oxides like TiO₂ and ZnO.

With regard to the preparation of the glass frits the same applies asalready mentioned above in connection with the preparation of the glassfrits in metal paste A.

The ratio of organic vehicle in metal paste B to the inorganic content(inorganic components; electrically conductive metal powder plus glassfrit plus optionally present other inorganic additives) is dependent onthe method of printing metal paste B and the kind of organic vehicleused, and it can vary. Usually, metal paste B will contain 53 to 95wt.-% of inorganic components and 5 to 47 wt.-% of organic vehicle.

The inorganic content of metal paste B comprises (b1) at least oneelectrically conductive metal powder selected from the group consistingof silver, copper and nickel, and (b2) 0.2 to 3 wt.-%, preferably 0.2 to2 wt.-% of glass frit. The inorganic content may further comprise otherinorganic additives, for example, solid oxides or compounds capable offorming solid oxides during firing of metal paste B. In an embodiment,the inorganic content of metal paste B consists of (b1) at least oneelectrically conductive metal powder selected from the group consistingof silver, copper and nickel, and (b2) 0.2 to 3 wt.-%, preferably 0.2 to2 wt.-% of glass frit.

Metal paste B is a viscous composition, which may be prepared bymechanically mixing the electrically conductive metal powder(s) and theglass frit with the organic vehicle. In an embodiment, the manufacturingmethod power mixing, a dispersion technique that is equivalent to thetraditional roll milling, may be used; roll milling or other mixingtechnique can also be used.

Metal paste B can be used as such or may be diluted, for example, by theaddition of additional organic solvent(s); accordingly, the weightpercentage of all the other constituents of metal paste B may bedecreased.

In step (2) of the process of the present invention metal paste B isprinted, in particular, screen printed in a grid pattern which comprises(i) thin parallel finger lines forming a top set of finger linessuperimposing the bottom set of finger lines and (ii) two or moreparallel busbars intersecting the finger lines at right angle. In anembodiment, the grid pattern is an H pattern with two parallel busbars.The parallel finger lines of the top set of finger lines so formed havea dry layer thickness of, for example, 3 to 30 μm and a width of, forexample, 25 to 150 μm. The total dry layer thickness of the finger lines(bottom plus top finger line dry layer thickness) is in the range of,for example, 10 to 50 μm. The busbars have a dry layer thickness of, forexample, 10 to 50 μm and a width of, for example, 1 to 3 mm.

The printed metal paste B is dried, for example, for a period of 1 to100 minutes with the silicon wafer reaching a peak temperature in therange of 100 to 300° C. Drying can be carried out making use of, forexample, belt, rotary or stationary driers, in particular, IR beltdriers.

Firing Step

The firing step (3) following steps (1) and (2) is a cofiring step. Itis however also possible, although not preferred, to perform anadditional firing step (1a) between steps (1) and (2).

The firing of step (3) may be performed, for example, for a period of 1to 5 minutes with the silicon wafer reaching a peak temperature in therange of 700 to 900° C. The firing can be carried out making use of, forexample, single or multi-zone belt furnaces, in particular, multi-zoneIR belt furnaces. The firing may happen in an inert gas atmosphere or inthe presence of oxygen, for example, in the presence of air. Duringfiring the organic substance including non-volatile organic material andthe organic portion not evaporated during the drying may be removed,i.e. burned and/or carbonized, in particular, burned and the glass fritsinters with the electrically conductive metal powder. Metal paste Aetches the ARC layer and fires through making electrical contact withthe silicon substrate. If metal paste B has fire-through capability, thebusbars etch the ARC layer and fire through making electrical contactwith the silicon substrate. In the preferred cases of metal paste Bhaving no or only poor fire-through capability the busbars remain as“non-contact” busbars (floating busbars, busbars which have not or onlyto a limited extent fired through the ARC layer) after firing, i.e. theARC layer survives at least essentially between the busbars and thesilicon substrate.

It is an additional advantage of the process of the present inventionthat the grid electrodes made thereby are distinguished by outstandingsolder adhesion.

EXAMPLES (1) Manufacture of Solar Cell

A solar cell was formed as follows:

(i) On the front face of a Si substrate (200 μm thick multicrystallinesilicon wafer of area 243 cm², p-type (boron) bulk silicon, with ann-type diffused POCl₃ emitter, surface texturized with acid, SiN_(x) ARClayer on the wafer's emitter applied by CVD) having a 30 μm thick fullplane aluminum electrode (screen-printed from PV381 Al compositioncommercially available from E. I. Du Pont de Nemours and Company) afront-side silver paste (PV159 commercially available from E. I. Du Pontde Nemours and Company, inorganic content without metal: 7 wt.-%, glassfrit content: 2 wt.-%) was screen-printed and dried as 100 μm wide andparallel finger lines having a distance of 2.25 mm between each other.Then a silver paste B1/B2 (B1 or B2 respectively) was screen printedsuperimposing the bottom set of finger lines as 100 μm wide and parallelfinger lines having a distance of 2.25 mm between each other and withtwo 2 mm wide and 11 μm/8 μm thick parallel busbars intersecting thefinger lines at right angle. All metal pastes were dried beforecofiring. Total thickness of the fingers after firing was 30 μm/25 μm.

The silver pastes B1/B2 comprised 85/81 wt.-% silver powder (averageparticle size 2 μm) and 15/19 wt.-% organic vehicle (organic polymericresins and organic solvents) plus glass frit (average particle size 0.8μm). The glass frit content of silver pastes B1/B2 was 0.5/2.0 wt.-%.Table 1 provides composition data of the glass frit types that have beenused.

(ii) The printed wafers were then fired in a Despatch furnace at a beltspeed of 3000 mm/min with zone temperatures defined as zone 1=500° C.,zone 2=525° C., zone 3=550° C., zone 4=600° C., zone 5=925° C. and thefinal zone set at 900° C. After firing, the metallized wafers becamefunctional photovoltaic devices.

Measurement of the electrical performance was undertaken. Furthermorethe laydown was measured.

(2) Test Procedures Efficiency

The solar cells formed according to the method described above wereplaced in a commercial I-V tester (supplied by h.a.l.m. elektronik GmbH)for the purpose of measuring light conversion efficiencies. The lamp inthe I-V tester simulated sunlight of a known intensity (approximately1000 W/m²) and illuminated the emitter of the cell. The metallizationsprinted onto the fired cells were subsequently contacted by fourelectrical probes. The photocurrent (Voc, open circuit voltage; Isc,short circuit current) generated by the solar cells was measured over arange of resistances to calculate the I-V response curve.

Table 2 provides an overview about the examples 1 and 2 (both accordingto the invention) and comparative example 3.

TABLE 1 Glass composition in wt. %: Glass type SiO₂ Al₂O₃ B₂O₃ PbO Bi₂O₃TiO₂ PbF₂ Glass in 22 0.4 7.5 46.8 6.8 5.8 10.7 paste A Glass in 28 4.78.1 55.9 3.3 paste B

TABLE 2 Total Grid 1^(st) 2^(nd) laydown Voc Isc Efficiency FFresistance Ex. layer layer (mg) (mV) (A) (%) (%) (mΩ) 1 PV159 B1 256612.2 8.11 15.62 75.9 18.7 2 PV159 B2 266 618.0 8.16 16.04 77.4 20.0 3PV159 PV159 438 612.7 8.15 15.13 73.8 21.9

1. A process of forming a grid electrode on the front-side of a siliconwafer having a p-type region, an n-type region, a p-n junction and anARC layer on said front-side, comprising the steps: (1) printing anddrying a metal paste A having fire-through capability on the ARC layer,wherein the metal paste A is printed as thin parallel finger linesforming a bottom set of finger lines, (2) printing and drying a metalpaste B over the bottom set of finger lines, wherein the metal paste Bis printed in a grid pattern which comprises (i) thin parallel fingerlines forming a top set of finger lines superimposing the bottom set offinger lines and (ii) two or more parallel busbars intersecting thefinger lines at right angle, and (3) firing the double-printed siliconwafer, wherein the metal paste A comprises an organic vehicle and aninorganic content comprising (a1) at least one electrically conductivemetal powder selected from the group consisting of silver, copper andnickel, and (a2) 0.5 to 8 wt.-% of glass frit, wherein the metal paste Bcomprises an organic vehicle and an inorganic content comprising (b1) atleast one electrically conductive metal powder selected from the groupconsisting of silver, copper and nickel, and (b2) 0.2 to 3 wt.-% ofglass frit, and wherein the inorganic content of metal paste B containsless glass frit plus optionally present other inorganic additives thanthe inorganic content of metal paste A.
 2. The process of claim 1,wherein the total content of the electrically conductive metal powder inmetal paste A is 50 to 92 wt.-%.
 3. The process of claim 1, wherein thetotal content of the electrically conductive metal powder in metal pasteB is 50 to 92 wt.-%.
 4. The process of claim 1, wherein the at least oneelectrically conductive metal powder in metal paste A is silver powder.5. The process of claim 1, wherein the at least one electricallyconductive metal powder in metal paste B is silver powder.
 6. Theprocess of claim 1, wherein metal paste B has no or only poorfire-through capability.
 7. The process of claim 6, wherein the glassfrit contained in metal paste B consists of at least one glass fritselected from the group consisting of (i) lead-containing glass fritswith a softening point temperature in the range of 571 to 636° C. andcontaining 53 to 57 wt.-% of PbO, 25 to 29 wt.-% of SiO₂, 2 to 6 wt.-%of Al₂O₃ and 6 to 9 wt.-% of B₂O₃, (ii) lead-free glass frits with asoftening point temperature in the range of 550 to 611° C. andcontaining 11 to 33 wt.-% of SiO₂, >0 to 7 wt.-% of Al₂O₃ and 2 to 10wt.-% of B₂O₃, and (iii) lead-free glass frits with a softening pointtemperature in the range of 550 to 611° C. and containing 40 to 73 wt.-%of Bi₂O₃, 11 to 33 wt.-% of SiO₂, >0 to 7 wt.-% of Al₂O₃ and 2 to 10wt.-% of B₂O₃.
 8. The process of claim 1, wherein the ARC layer isselected from the group consisting of TiO_(x), SiO_(x), TiO_(x)/SiO_(x),SiN_(x) or Si₃N₄ ARC layers.
 9. The process of claim 1, wherein anadditional firing step (1a) is performed between steps (1) and (2). 10.The process of claim 1, wherein the printing in steps (1) and (2) isscreen printing.
 11. A front-side grid electrode produced according tothe process of claim
 1. 12. A silicon solar cell comprising a siliconwafer having an ARC layer on its front-side and the front-side gridelectrode of claim 11.